Definitely go during the week. Yes, this may mean pulling your son out of school for a day or two, but you're not going to get a feel for the campus by visiting on Sunday morning. (My parents forbade me to miss even one second of high school, so I didn't get to do things like sit in on lectures.)
If there is a freshman core curriculum, he should sit in on a lecture or two. If he knows or has a good idea what he wants to major in, he should sit in on <whatever it is> 101.
Let him off his leash. (Can you tell I had overprotective parents?) Let him meet and chat with other prefrosh and current students without mom and dad hovering around. He'll get a better idea of campus life that way - nobody will say "This is a huuuuuuge party school!" in front of mom and dad. Meanwhile, you might chat with other parents, take a walk around campus, or check out the city/town where the school is located - look at off-campus housing, restaurants, cultural opportunities like museums, etc.
As for early decision, keep in mind that it's binding. Your son should only apply early decision if he knows he wants to go to ABC U and nowhere else. Early action is non-binding. But if you and he are comfortable pursuing early decision or early action, go for it! I applied early action, and even though I sent in a few regular applications too, it was a huge load off my mind when I got my acceptance letter and I knew I didn't have to stress about college applications any more.
Good luck with the college hunt.